1. Field of the Invention
This invention relates generally to the field of subtalar joint and first metatarsal-phalangeal implants for treating foot conditions including flat feet, adult posterior tibial tendon dysfunction and metatarsophalangeal joint dysfunction.
2. Description of the Related Art
Pes valgo planus, or flat foot, is a common condition where the arch of a foot is weakened and is unable to properly support the weight of the body. With a flat foot, shock absorption is reduced and misalignment of the foot occurs. These changes may eventually result in foot and ankle pain, tendonitis, plantar fasciitis and hallux valgus, hallux limitus and functional disorders of the knees, hips and back. Although there are several causes of flat feet, one frequent cause is excessive motion in the subtalar joint of the foot.
As early as 1946, surgeons have been attempting to apply the arthroereisis concept to the subtalar joint. Arthroereisis is a surgical procedure for limiting motion in a joint in cases of excessive mobility. One early method was to remedy abnormal excursion of the talus on the calcaneus with the talus contacting the floor of the sinus tarsi by using an “abduction block” procedure. During the abduction block procedure, a wedge-shaped bone graft was impacted into the anterior leading edge of the posterior facet of the calcaneus. Impacting such a bone graft prevented excessive inferior displacement of the talus upon the calcaneus, thus limiting the amount of excess pronation of the subtalar joint.
A pronation limiting osteotomy in the form of a lateral opening wedge of the posterior facet was developed for treatment of “flatfoot” in cerebral palsy patients in 1964. In order to prevent interfering with subtalar joint motion, a wedge-like bone graft was used to improve the weight-bearing alignment of the calcaneus. In 1970, an accessory bone graft placed in the sinus tarsi was developed as a corrective procedure. Later, the bone graft was replaced with a silastic plug. As early as 1976, a high molecular weight polyethylene plug was developed. The plug is cemented into the calcaneal sulcus against a resected portion of the posterior calcaneal facet. This procedure, known as “STA-peg” (subtalar arthroereisis-peg), is a commonly used subtalar joint arthroereisis procedure. STA-peg does not block excessive pronation, but rather alters the axis of motion of the subtalar joint.
In addition, in 1976, a high molecular weight, polyethylene, threaded device known as a “Valenti Sinus Tarsi Arthroereises Device” was invented. The procedure used to implant the Valenti device is commonly referred to as the “Valenti” procedure. Unlike the STA-peg procedure, the Valenti procedure is an extra-articular procedure that involves placing the Valenti device into the sinus tarsi to block the anterior and inferior displacement of the talus. Such placement of the Valenti device does not restrict normal subtalar joint motion, but does block excessive pronation and resulting sequelae. The Valenti device has a frusto-conical shape and threads on the outer surface of the device, which allow it to be screwed into the sinus tarsi. Because of the shape of the Valenti device, the greater the penetration of the device into the sinus tarsi, the more the sinus is dilated and the more calcaneal eversion is eliminated.
However, several problems reduce the desirability of the Valenti procedure and device. Because of its frusto-conical shape and the manner in which it is inserted, the Valenti device is difficult to precisely position in the subtalar joint and difficult to ensure that the proper amount of calcaneal eversion has been eliminated. Furthermore, it is generally difficult to locate the device properly within the tarsal canal because the implant must be threaded at least 3 to 5 millimeters medial to the most lateral aspect of the posterior facet for correct placement. Because of its polyethylene construction, the device cannot be imaged using radiography (X-ray) to determine whether the proper position has been achieved.
More recent attempts to control subtalar motion in the hyperpronated foot include the Maxwell-Brancheau arthroereisis (MBA), the Kalix subtalar prosthesis and the Futura arthroereisis. The MBA is a titanium alloy implant where the implantation procedure involves insertion “trial” implants to determine the proper size of the actual implant used. The MBA implant procedure requires either general anesthesia or local anesthesia with sedation. It also requires up to a ¾ inch incision on the lateral portion of the foot. The MBA implant uses a metal guide pin for positioning the implant. The guide pin must be positioned with extreme care to prevent damage to the calcaneus. A two-week period of crutch use and foot immobilization typically follows the procedure. The Kalix implant is a cone-shaped implant with limited expansion ability. The operator can use a double screwdriver to increase the diameter of the implant. The Kalix implant requires two weeks of non-weight bearing and three to four weeks of immobilization following implantation of the device.
Another site of frequent foot problems is the first metatarsal-phalangeal joint. The first metatarsal-phalangeal joint (MTP) is a complex joint of the foot where bones, tendons and ligaments work together to transmit and distribute the body's weight, especially during movement. Bunions are the first MTP joint disorder most frequently treated by podiatric surgeons. First-line treatment involves educating patients about the condition and evaluating their footwear. Providers can direct their patients to wear wider, low-heeled shoes, use bunion pads, apply ice and take over-the-counter analgesic medications. These options are designed to relieve pain and make it easier to walk and engage in physical activities, but they do not address the underlying cause of bunions.
Bunions usually occur from inherited faulty biomechanics that put abnormal stress on the first MTP joint. Contrary to popular belief, bunions are aggravated, not caused, by shoes. Various non-surgical approaches can help prevent aggravation of bunions and other MTP-related problems. For some patients, non-surgical treatment is sufficient, but surgical intervention is considered if the bunions are progressive or if non-operative treatments provide inadequate improvement.
Bunion surgery is performed to repair tendons and other soft tissue and remove a small amount of bone. Procedures to correct more severe bunions may involve removal of the bump or minor realignment of the big toe joint. The most severe and disabling bunions often require extensive joint realignment, reconstruction, implants or joint replacement. Significant morbidity and recuperation time is required for such procedures.
First MTP-related problems also occur from repetitive trauma to the area and from arthritis. Over time, active persons can put continuous stress on the first MTP joint that eventually wears out the cartilage and lead to the onset of arthritis. This condition, known as hallux rigidus, causes loss of movement and pain in the joint. In most situations, non-operative treatments can be prescribed to provide relief, but those with advanced disease might need surgery, especially when the protective covering of cartilage deteriorates, leaving the joint damaged and with decreased range of motion. Again, significant morbidity results from these procedures and an extended recovery time is required.